59575-74-5Relevant academic research and scientific papers
Semiexhaustive determination of rate parameters for dissociative axial ligand substitution in 24 ferrous dimethylglyoxime complexes involving 7 different axial ligands
Chen, Xuening,Stynes, Dennis V.
, p. 1173 - 1182 (2008/10/08)
Syntheses, visible spectra, and kinetic data are presented for 24 of 28 possible complexes Fe(DMGH)2LT (L or T = methylimidazole (MeIm), pyridine (py), tributylphosphine (PBu3), tributyl phosphite (P(OBu)3), benzyl isocyanide (BzNC), tosylmethyl isocyanide (TMIC), and carbon monoxide). Axial ligand substitution reactions proceed via a dissociative mechanism in all cases. Dissociative rate constants spanning 8 orders of magnitude depend upon a leaving group order py > MeIm > P(OBu)3 > PBu3 ≥ CO > BzNC ? TMIC and a trans labilizing order py > MeIm > PBu3 > P(OBu)3 > BzNC ≥ TMIC > CO. The trans effect on MeIm and py dissociation correlates with the MLCT band in the visible spectrum, indicating that weakening of π bonding to the trans ligand constitutes a significant part of the barrier for dissociation. Relative rates of addition to the pentacoordinate intermediates cover about a factor of 10 and are independent of the trans ligand. The order of addition is similar to that found in hemes: MeIm ? BzNC ? TMIC > PBu3 ? P(OBu)3 > CO. Rates of dissociation of pyridines with substituents 4-CN, 3,4-Me2, and 4-NMe2 correlate with ligand basicity. Trans effects with increased donor strength of substituted pyridines are mildly accelerating for loss of σ donors and mildly decelerating for loss of CO. Relevance to other low-spin d6 systems including hemes and Cr(0) complexes is discussed.
